Ballast apparatus



Nov. 21, 1961 R. E. HUME ETAL BALLAST APPARATUS 3 Sheets-Sheet 1 FiledFeb. 29, 1960 W 7 m B z 2 3W5? r 2 0 H 0 2 w mLr /A 2% 3 8 5 M 4 a Q P w"x m Nov. 21, 1961 R, E. HUME ETAL BALLAST APPARATUS 3 Sheets-Sheet 2Filed Feb. 29, 1960 .n My

Nov. 21, 1961 R. E. HUME ETAL BALLAST APPARATUS 3 Sheets-Sheet 3 FiledFeb. 29, 1960 ew am a mhmisrjivimiwim aoaifiawil 320 530 340 .950 36057a 38a 53a 400 4/0 420 430 [/7 1 81? tors @m M 5 r we wow 2 UnitedStates ate 3,010,950 BALLAST APPARATUS Roger E. Home and DanielLovinger, Danville, Ill., as-' signers to General Electric Company, acorporation of New York Filed Feb. 29, 196tl, Ser. No. 11,873 .18Claims. (Cl. 315-138) This invention relates to ballast apparatus andmore particularly to ballast apparatus including a high reactanceballast transformer having a lead secondary winding'coupied with aprimary winding.

When are discharge devices, such as fluorescent lamps are operated onalternating current, a high reactance transformer is used to provide ahigh open circuit volt age for starting the lamps, 'a relatively lowervoltage'for tend to cause the transformer to have a relatively highleakage reactance.

The effect of leakage reactance is to reduce the coupling between theprimary and secondary over the coupling that would normally be obtainedif the primary and secondary windings were tightly coupled. Conventionalmeans used for supplying a path or shuntfor the leakage flux are toprovide a space in the coil receiving window between the primary and thesecondary windings for the flow of leakage flux or to provide a metallicshunt and air gap between the primary and secondary windings, or adistributed path along the length ofthe secondary coil. Since themagnetic cores generally used in ballast transformer constructions areof the shell-type having an elongated central winding leg on which theprimary and secondary windings are arranged and a pair of outer yokelegs surrounding the primary and secondary winding to a return path forthe magnetic flux, it has been the common practice to provide legportions on either the center Winding-leg or the outer yoke'legs to formmagnetic shunts between the winding, with the legs being so formed thatan air gap is provided in the shunts. In some prior art constructionsbundled shunts have been interposed between a primary Winding to providea path for the leakage flux for a lag secondary winding. Such a priorart construction employing transverse bundled shunts and a series airgap is described in U.S. Patent No. 2,568,553-- Mauerer. Since thepermeability of iron or other magnetic metals used in transformer coresis much better than that of air, it is possible with a metallic shunt toprovide the same leakage reactance with a shorter core length ascompared with an air path between the primary and secondary windings ora distributed leakage along the secondary coil.

In the past it was considered undesirable to permit the flux density ina metallic shunt to exceed the point at which it would saturate. When ashunt saturate-s, a lower inductance or leakage reactance results in thecircuit. Therefore, an air gap is ordinarily used to control the fluxdensity in the metallic shunt under high current conditions and toprevent the shunt from reaching a point of saturation. In other words,since the iron might introduce a nonlinear eifect in the magneticcircuit which might cause the shunt to operate in its saturation region,heretofore, this possible effect was avoided by the use of the air gapin the shunt. The high reluctance provided by the air gap rendered thereluctance of the leakage path 23 substantially linear since the air gapcontributed most of the reluctance.

The present invention is particularly concerned with a high reactanceballast transformer which includes a lead secondary winding coupled on amagnetic core with a primary winding. A lead secondary, as the term isused herein, is a secondary winding of a ballast transformer whichsupplies a current to a leading power factor load. In other words, it isa secondary in which a leading current flows during operation due to thepredominantly capacitive reactance of the secondary circuit of theballast apparatus. In such a circuit the lamp is supplied with theleading current flowing in the secondary of the high reactancetransformer. Since the lead secondary winding carries a leading current,the flux produced by it in the central winding leg is more or less inphase with the exciting flux produced by the primary winding. The fluxwhich flows through the shunt or leakage path interposed.

between the lead secondary winding and the primary winding is equal tothe vector difference of the primary flux and the total flux under thesecondary, and generally is relatively small in magnitude. However, theflux produced by a lag secondary is substantially in phase opposition tothe exciting flux induced in the central winding leg by the primarywinding. Consequently, the flux which flows in the shunt-between alagging secondary and a primary is the vector sum of the fluxes producedby the windings. Thus, the magnitude of the flux in a shunt between alag secondary winding and the primary winding is relatively greater. Inorder to prevent the flux density in metallic shunts from reachingsaturation under operating conditions it will be seen that the crosssectional area of shunts used between lag secondary windings and primarywindings are necessarily greater than those used between lead secondarywindings and primary windings in comparable magnetic circuits.

In the past constructions of high reactance transformers utilizingmetallic shunts, the leakage reactance does not change to anyappreciable degree between open circuit and normal operating conditions.In other words, the leakage reactance of conventional high leakagereactance transformers has been essentially linear, any nonlinearitywhich might be introduced because of the high permeability of the ironhaving been effectively suppressed by air gaps. I Y

Several difficult problems have been encountered in providing suitableballast apparatus for series lead circuits of desired small size andexpense without a sacrifice in lighting efficiency or ballastperformance. For ex ample, conventional constructions of high reactanceballast transformers were found to produce the condition of instabilityin fluorescent lamps and especially with respect to low temperatureoperation of lamps in series lead circuits. This instability resulted ina low frequency oscillation or variation in the visible light output ofthe lamps which produced an objectionable flickering. Conventionalexpedience for increasing the leakage reactance in the ballastingsecondary winding have not produced satis factory results. In someapplications insufiicient space was available to increase the leakagereactance by adding turns or by providing additional spacing between theprimary and secondary windings. Attempts to reapportion the magnetizingand leakage reactance of the secondary winding failed because theinstability was still present.

Accordingly, an object of the invention is to provide an improved highreactance ballast transformer and ballast apparatus for starting andballasting a gaseous discharge lamp, such as a fluorescent lamp, havingstable and improved operating characteristics.

Another object of the invention is to provide an improved high reactancetransformer and ballast apparatus wherein the leakage reactance iselfectively utilized to improve the waveform of the lamp current.

It is still another object of the invention to provide an improvedballast apparatus for series lead circuits operating a pair offluorescent lamps.

A more specific object of the invention is to provide an improvedballast apparatus that will provide a leakage reactance at an earlyportion of each half cycle of the alternating current supplied to thelamp, which is decreased during a later portion of each half cycle.

It is still a further object of the present invention to provide a highreactance ballast transformer of improved versatility that will permit aballast apparatus of smaller size to be constructed without a sacrificein lighting efficiency or ballast performance.

In accordance with the invention, a ballast apparatus is provided forstarting and operating one or more lamps in a series lead circuit inwhich a nonlinear shunt is utilized between the lead secondary windingand primary of a high leakage reactance transformer to introduce anincreased leakage reactance during the early portion of each half cycleof the secondary current and a decreased leakage reactance during alater portion of each half cycle. Preferably, the ballast apparatus, inaccordance with the invention may be used for operating a pair offluorescent lamps in a series lead circuit arrangement.

In another aspect of the invention, a high reactance ballast transformeris provided having a magnetic core with a central elongated winding leg,on which the primary winding and at least one lead secondary winding arelocated in an end to end relationship. A pair of outer yoke legs aredisposed on both sides of the central elongated winding leg to provide aclosed path for the magnetic flux and to form a coil receiving windowfor the transformer windings. In the coil window space between theprimary winding and the lead secondary winding we have inserted anonlinear shunt having a suificiently restricted cross-sectional area soas to cause flux saturation in the shunt during each half cycle of thesecondary current. The term nonlinear shunt. is used herein to denote ashunt (1) that is interposed between a primary winding and a secondarywinding in a magnetic circuit, (2) that saturates and desaturates duringeach half cycle of the secoridary current, and (3) that provides aleakage reactance which substantially varies during each half cycle ofthe secondary current.

Although in the' illustrated embodiments of our invention we haveemployed transverse shunt lamination inserts between the primary and thelead secondary windings, it will be apparentthat relatively thinprojections extending laterally and formed integrally with the centralwinding leg or the outer yoke legs can be used to provide a nonlinearshunt in accordance with the broader aspects of the invention.

The subject matter which we regard as our invention is set forth in theappended claims. Theinvention itself, however, together with furtherobjects and advantages thereof may be better understood by referring tothe following description taken in connection with the accompanyingdrawings in which:

FIG. 1 is a plan view of a high reactance ballast transformer in whichthe invention is embodied;

FIG. 2 is a sectional view along the plane 22 of the high reactancetransformer of FIG. 1;

FIG. 3 is a perspective view of a shunt lamination used in theillustrated embodiments of the invention;

FIG. 4 is a plan view of another high reactance transformer embodyingthe invention;

FIG. 5 is a plan view of a high reactance transformer illustrating anembodiment of the invention with a portion of the winding cut away;

FIG. 6 is a schematic circuit diagram of a ballast apparatus utilizingthe high reactance transformer of FIG. 1;

FIG. 7 is a schematic circuit diagram of a ballast apparatus utilizingthe high reactance transformer of FIG. 4;

4 FIG. 8 is a schematic circuit diagram of a ballast apparatus utilizingthe high reactance transformer of FIG. 5; FIG. 9 illustrates the twocurves showing the waveform of the lamp current flowing in a series leadballast circuit, curve A representing the waveform of the lamp suppliedby a ballast apparatus employing nonlinear shunts in accordance with theinvention and curve B representing the waveform of the lamp currentsupplied by a conventional ballast in which a substantally constant leakage reactance was provided; and 7 FIG. 10 presents two curves, curve Crepresenting the primary flux density in lines per square inch plottedagainst the shunt flux density in lines per square inch and curve Drepresenting the primary flux density in lines per square inch plottedagainst leakage reactance in ohms. Referring to FIG. 1, there is shown ahigh reactance transformer generallyidentified by the numeral 11. Thetransformer 11 is formed of a magnetic core 12 including a centralwinding leg 13 and the outer yoke legs 14, 15 arranged respectively onopposite sides of the center leg member 13. The magnetic core 12 iscomprised of stacks of a plurality of relatively thin laminations ofelectrical steel, there being three lamination pieces per lainationlayer. Mounted on the elongated central winding leg 13 within a coilreceiving windows 16, 17 defined by the yoke legs 14, 15 are the primarywinding H and the lead secondary winding 19. The cathode heatingwindings are shown schematically in FIG. 6, one being included in theprimary winding 18 and two being coupled in a secondary transformerrelationship therewith. The laminations of the central winding leg 13and outer yoke legs may be held together by any suitable means, such asthe spring clamps 2h, 21 which are shown. disposed at the opposite endsof the magnetic core 12. It will be seen that the clamps 20, 21 alsohold the outer yoke legs 14, 15 in an abutting relationship with thecentral winding leg 13. In the coil receiving windows l5, 17, twotransverse shunt laminations 22 spaced from the outer yoke legs 14-, 15are interposed between the pri rnary winding 18 and secondary winding19. 'Although as shown in the sectional view of FIG. 2 and in theperspective view of FIG. 3, the lamination 22 has a G- shapedconfiguration, this particular configuration was used since itfacilitates assembly, the inwardly extending. projections 23, 24preventing the shunt laminations 22 from falling out during themanufacturing operation.

Referring now to FIG. 2, it will be seen that the trans verse shuntlaminations 22 are disposed vertically in the coil receiving window andspaced from the outer yoke legs l4, l5 and the central winding leg 13.The shunt laminations 22 are retained on the central winding leg 13 bythe inwardly extending projections 23, 24. Although we have shown anddescribed shunt laminations in the illustrative embodiment of ourinvention, it will be readi- 1y apparent to those skilled in the artthat the nonlinear shunt in accordance with the invention may beintegrallyformed on the outer yoke legs 14, 15 on the central wind ingleg 13 or on both the yoke legs l4, l5 and the central winding leg 13,each providing a portion thereof. The lamination 22 was fabricated ofelectrical, i.e., magnetic, strip sheet steel for reasons of convenienceand economy- However, it will be appreciated that magnetic alloys can beused to provide the variable leakage reactance in accordance with thisinvention.

As shown in FIG. 4, a magnetic core 25 of the high reactance baliasttransformer 26 comprises an elongated central winding leg 27 and a pairof yoke legs 28, 29. The spring clamps 30, 31 hold the outer yoke legs28, 29 in an abutting relationship with the central winding member 27.It will be noted that the yoke legs 28, 29 are provided with integrallyformed metallic shunts 32, 33, which are spaced from the central windingleg 27 to provide air gaps 34, 35. As viewed in FIG. 4, a lag secondarywinding 36 is disposed to the left of the metallic shunts 32, 33 and aprimary winding 37 is disposed to the right of the shunts 32, 33. A leadsecondary winding 38 is mounted on the central winding leg 27 to theright of the primary winding 37. The air gaps 34, 35 between the shuntand the central winding leg 37 provide a high reluctance which rendersthe magnetic characteristic of the shunting circuit substantially linearsince most of the reluctance of the circuit is in the air gaps 34, 35.Interposed between the lead secondary winding 38 and the primary winding37 are a pair of nonlinear shunt laminations 39 being spaced from theouter yoke legs 28, 29. In accordance with the invention, the operationof this ballast transformer 26 will be more fully described inconnection with the description of its associated ballast apparatusillustrated in FIG. 7.

In FIG. another form of our invention is shown embodied in a highreactance transformer 40 of'the shell-' type in which two primarywindings 41, 42 aredisposed at opposite ends of a lead secondary winding43. Like the ballast transformers 11, '26, shown in FIGS. 1 and-4, thetransformer of FIG. 5 is of the shell-type having a magnetic core 44with an elongated central winding leg 45 and a pair of outer yoke legs46, 47 disposed in an abutting relationship on opposite sides of thecentral winding leg 45 to form coil receiving windows 48, 49. Springclamps 50, 51 hold the outer yoke legs in an abutting relationship withthe central winding leg 45.

The high reactance of the transformer 40 is provided by the distributedleakage of magnetic flux between the elongated central winding leg 45and the yoke legs 46, 47 and the nonlinear shunts of the invention. Inac cordance with the invention, two laminations 22 of the type shown inFIG. 3 are included at each end of the lead secondary winding 43 toserve as a nonlinear shunt between the primary windings 41', 42 and thelead secondary winding 43. It will be seen that the shunt laminations 22of this invention do not add appreciably to the length of the magneticcore 44 and are spaced from the outer yoke legs 46,47.

Three spaced transverse slots 52, 53, 54 are formed in the centralwinding leg 45 and form bridged gaps serving the purpose of preventingsaturation of the secondary winding magnetic circuit in order to keepcore losses at a minimum and to prevent distortion of the lamp currentwaveform. Thus, the advantages derived by using bridged gaps as shown inFIG. 5 and also in the embodiments of FIGS. 1 and 4 can be realized inconjunction with the advantages of this invention.

The specific circuit connections of the high reactance ballasttransformers 11, 26, 40 of FIGS. 1, 4- and 5 will now be considered inconnection with the schematic circuit diagrams of FIGS. 6, 7 and 8 inwhich the transformers are schematically illustrated. In the circuitdiagrams it will be noted that the nonlinear shunts of this inventionare identified symbolically in the circuit'using two parallel linescrossed by a slant line.

Referring now to FIG. 6, the ballast apparatus is enclosed in a dashedrectangle 56 and includes two of the high reactance transformers 11,11', shown in FIG. 1. The ballast apparatus is intended for starting andoperating two serially connected 96 PG17 type power groove lamps. Thepower groove lamps are hot cathode lamps which have an unusually highwattage rating for a given starting voltage rating. The correspondencebetween the high reactance ballast transformer shown in FIG. 1 and thetransformers shown schematically in FIG. 6 is indicated by using likenumerals to identify the corresponding parts. The high reactance ballasttransformers 11, i

11 include primary windings, 18, 18', secondary windings 19, 19',magnetic cores 12, 12 and nonlinear shunts comprising the laminations 2222, A heating winding 59 is connected to filament 60 of lamp 57, andheating winding 61 is connected to filaments 62, 63 of lamps 57, 58.Heating winding 64 which is connected to filament 65 may be anextension, as shown, of the primary winding 18'.

Two external input leads 66, 67 are provided for connection to a, sourceof alternating voltage (not shown), for example, volts at a frequency of60 cycles and are connected across primary winding 18'. Primary winding18 is also connected to the external input leads 66, 67 by conductors68, 69. It will be noted that the two secondaries 19, 19 are connectedin series circuit relationship, the secondary winding 19" being inautotransformer relationship with the primary winding 18. A startingcapacitor 71 is connected between the high voltage lead or output lead72 and an intermediate voltage lead 70 which is also connected to theheating winding 61. A second output lead 73 joins one end of the primarywinding 18 with filament 65 of lamp 58. Connected in series circuitrelationship with the secondary winding 19 is a capacitor 74 selected tohave sumcient capacity to deliver to the two lamps 57, 58 the combinedoutput of the currents developed by the two transformers 11, 11 and toprovide a net capacitive reactance in the secondary circuit.

When the input leads 66, 67 are connected to a suitable source ofalternating current (not shown), the total open circuit voltage, whichis the sum of the input voltage across the primary windings 18, 18' andthe individual voltages of the secondary windings 19, 19 is impressedacross the serially connected lamps 57, 58. Due to the connection of thestarting capacitor 71 across the lamp 57, substantially all of theoutput voltage appears in the first instance across the lamp 58. Also,at the same time the filaments 60, 65 are being quickly heated toelectron emitting temperatures and soon thereafter, lamp 58 is ignited.The discharge current in the lamp then flows through the startingcapacitor '71 thereby pro ducing a relatively high voltagedrop in thecapacitor, which voltage drop is impressed across lamp 57 and causeslamp 57 to form an arc discharge between its heated filaments 60, 62.After the lamp 57 has fired, the starting capacitor '71 is effectivelyshort circuited by the lamp 57. I

In prior constructions of the high reactance ballast transformers,difficulties were encountered in the lamp stability as attempts weremade to reduce the lengthof the magnetic core in order to meet designrequirements. In prior designs of the high reactance transformer used inthe ballast apparatus shown in FIG. 6, a space between the primary andthe secondary was provided. We found that the length of the magneticcore could be effectively reduced and in addition a very significantimprovement in the operating characteristics of the ballast circuitcould be obtained by using the nonlinear shunts of this invention.

An important advantage derived from the use of nonlinear shunts betweenthe primary and lead secondary windings in a series lead ballast circuitof the type shown in FIG. 6 lies in the improvement of lamp currentwaveform. We discovered that the lamp current waveform is improved ifthe leakage reactance resulting from the leakage flux in the magneticcircuit were high during an early portion of each half cycle duringwhich the lamp reignites. A further improvement in lamp current waveformcan be obtained if the leakage reactance is at a minimum during a laterportion of the half cycle of each half cycle of the lamp current.

Referring now to FIG. 9, the current waveform defined by the solid line,curve A, represents the waveform obtained on an oscilloscope for thelamp current in the circuit of FIG. 6, in which the high reactanceballast transformers 11, 11' employed nonlinear shunts 22, 22' inaccordance with the invention. The current waveform defined by thedashed lines, curve B, represents the lamp current in the same ballastcircuit in which an air space forming a linear shunt was providedbetween the primary windings, 18, 18' and the lead secondaries 19, 19.

It was found that the low current values occurring during the initialperiod of each half cycle were primarily responsible for lampinstability and that this low current flow condition at each half cyclewas aggravated as the temperature of the lamp decreased. Thus, as anexample, lamps operated in the vicinity of an air conditioning outletwould flicker as a result of the low current flow condition. Wediscovered that a nonlinear shunt employed in accordance with theinvention provided a higher value of leakage reactance at the start ofeach larnp' current half cycle and thereby a higher current at thattime. This isevidenced by the substantially increased values of thecurrent flowing during the initial period of the cycle as shown by curveA of FIG. 9. It was found that this ballast transformer, using thenonlinear shunts in accordance with the invention did not experienceinstability when used for air conditioning applications.

Another advantage resulting from the use of nonlinear shunts inaccordance with the invention is further evident from the currentwaveform during the later part of each half cycle. The low leakagereactance provided by the nonlinear shunt during the later part of eachhalf cycle when the shunt has become saturated allows an increasedcurrent to flow, particularly the third harmonic component. In otherwords, the nonlinear shunt of the invention introduces a variableleakage reactance in ballast circuits. A high leakage reactance isprovided early in each lamp current half cycle, and a lesser amount ofleakage reactance is provided at a critical time in the later portion ofeach half cycle when the third harmonic current can be utilized toimprove the operating performance of the ballast transformer.

The advantages of the invention may also be obtained in a series leadballast circuit of the type shown in FIG. 7 in which the high reactancetransformer 26 of FIG. 4 is employed in the circuit to operate a pair offluorescent lamps 75, 76 in which the filament leads (not shown) areshort circuited inside the lamp base and which are known generally inthe trade as instant start lamps. The ballast apparatus as shownschematically in FIG. 7 is enclosed in a dashed rectangle 77, whichrepresents the ballast apparatus case. The correspondence between thehigh reactance ballast transformer of FIG. 4 and the 1 schematic diagramshown in FIG. 7 is indicated by using like reference numerals toidentify like parts. A primary winding 37 is connected across a pair ofinput leads 78, 79 which are adapted for connection to an alternatingcurrent source (not shown). A first secondary Winding 35 and a secondsecondary winding 38 are mounted on a magnetic core 25. In accordancewith the invention, a nonlinear shunt 39 is interposed between theprimary winding 37 and the second secondary winding 38. Between thefirst secondary winding 36 and the primary winding 37, a metallic linearshunt 86 is interposed. The linear shunt 30 symbolically represents theshunt projections 32, 33 and the series air gaps 34, 35 which are shownin FIG. 4. The inductive reactance of the first secondary winding 36 isrelatively greater as compared with the capacitive reactance ofcapacitor 8-1 so that the current in the secondary will lag, andtherefore the first secondary 3a is a lag secondary winding. Since thesecond secondary 38 supplies a leading current during normal operationof the ballast apparatus, the nonlinear shunt 39 is inserted between theprimary winding 37 and the secondary Winding 38.

From the circuit connections of the ballast apparatus of FIG. 7 it Willbe be readily seen that lamp 75 is connected across the primary winding37 and the first or starting secondary winding 36 while lamp 76 isconnected across the first secondary winding 36 and the second orballasting secondary winding 33. When an alternating voltage is appliedacross the input leads 7 8, 79, the applied voltage combined with thevoltage across the first secondary winding 36 appears across lamp 75 anda lagging current fiows in the circuit of lamp 75. As is well-known inthe art, the lagging current causes a shift of the phase of the voltagein the first secondary winding 36 so that it becomes generally additivewith the voltage in the second or ballasting secondary 38. This combinedvoltage appears across lamp 7 6 and is sufiicient to ignite it. Whenboth lamps have ignited, the ballast circuit operates as a series leadcircuit, including the primary winding 37 and the ballasting winding 38.The starting windings 36 is of relatively high impedance and is more orless excluded from the operating circuit, drawing only a very smallcurrent. During normal operation the path of current flow is from inputlead 79 through lead 83-, the lamp 76, the secondary winding 38, thecapacitor 81, lamp 75 and lead 8t} to input lead 73.

It was found that the nonlinear shunts significantly improved lampstability and the performance characteristics of the ballast apparatusshown in FIG. 7. Thus, the advantages of the invention can be realizedwhere lead and lag secondary windings are disposed in an end to endrelationship with a primary winding in a series lead ballast circuit.

Referring now to FIG. 8, a ballast apparatus employing the highreactance transformer 40 of FIG. 5 is illustrated for operating a pairof hot cathode lamps 34-, 85. As shown in FIG. 8, the primary windings41, 42 are arranged on a magnetic core 44, the primary winding 41 beinglocated at one end of the secondary winding 43 and the other primarywinding 42 being located at the other end of the secondary winding 43.The two primary windings 41, 4'2 and the secondary winding 43 are showndisposed on the magnetic core 44. The nonlinear shunts 86, 87, inaccordance with the invention, are placed on opposite sides of thesecondary winding 43 so they are interposed between the secondarywinding 43 and the primary windings 41, 42. The nonlinear shunts 36, 87are comprised of a pair of the laminations shown in FIG. 3. Except forthe single ballast transformer 40' being used with split primary windingarrangement, the ballast circuit of FIG. 8 is generally similar with thecircuit shown in FIG. 6.

The correspondence between the high reactance transformer shown in FIG.5 and the ballast apparatus of FIG. 8 is indicated by the use of likereference numerals to identify like parts. As shown in FIG. 8, theballast apparatus is enclosed by the dashed rectangle 88, whichschematically represents the ballast case. It will be noted that thesecondary winding 43 is in auto-transformer relationship with theprimary winding 42. A capacitor 89 is connected in series with thesecondary winding 43 and with lamps 84, 85. Another capacitor 90 isprovided as an aid in starting and is connected in parallel with thelamp 84. A first output lead 91 and a second output lead 92 areconnected across the serially connected lamps 84, 35. Two input leads93, 94 are provided for connection with an alternating current source(not shown). Current for heating the filaments 95, 96, 97, 98 areprovided by the heating windings 99, 101, 102.

Whenan alternating voltage is applied to the input terminals 33, 94, astepped up voltage comprising the voltage of secondary 43 added to theinput voltage appears across the output leads of ballast transformer 40.Due to the starting capacitor 98', this voltage is first applied acrosslamp to start it. Once lamp 35 starts, most of the voltage then appearsacross lamp 34 to start it. Once both lamps are started, the ballastapparatus of FIG. 8 operates as a series lead circuit in the same manneras the circuit shown in FIG. 6.

By splitting the primary winding into two primary windings ll, 42, ithas been found possible to derive the benefit of a reduced primarywinding temperature during normal'operation ofthe lamps and yet maintainthe stability, the improved performance characteristics, and otheradvantages which are realized by constructing a ballast transformer withnonlinear shunts of this invention.

As a specific illustrative example of the invention, the ballastapparatus of FIG. 6 was constructed for starting and operating two 96PGl7 power groove lamps. The apparatus utilized two of the highreactance transformers 9 11 shown in FIG. 1 and was housed in a ballastcase having the following outer dimensions; 19 inches long, 3% incheswide and 2% inches high. Each high reactanee transformer 11 was 6%inches long, 2 ,5 inches wide and 2% inches high. The outer yoke legs'14, of the magnetic core 12 were formed by laminations .025 inch thickand were stacked to a height of 1% inches. The center winding leg 13consisted of a stack of laminations stacked to a height of 1% inches.Two C-shaped laminations 22 of the type shown in FIG. 3'having athickness of .025 inch were inserted between the primary winding 18 andthe secondary winding 19. The primary winding had 370 turns of number 20copper wire and the secondary winding had 807 turns of number 20 copperwire. The heating windings 59, 61, 64 had 12, 13, 13 turns respectively.The capacitive reactor had a capacitance of 6.7 microfarads. Thestarting capacitor had a capacitance of .075 microfarad. The transformerdeveloped an open circuit voltage of 47 8 volts (root mean square)across both lamps. The power factor with both lamps operating was foundto be approximately 98.9.

In order to demonstrate the advantages derived by the ballast apparatusof the invention, the ballast apparatus described in the illustrativeexample was compared with an identical ballast in which an air space wasprovided between the lead secondary winding and primary winding inaccordance with the conventional practice.

The performance characteristics of the two ballasts are summarized inTable I.

From the data presented in Table I, it will be noted that forsubstantially the same light output of 91.4 and 92 percent, the ballastwith the nonlinear shunts require less power to operate the power groovelamps. The lower values of primary current and lamp current, in additionto the improved lamp power factor, make it possible to obtain anappreciable reduction in size and cost for given performancecharacteristics as compared with a comparable ballast in which thenonlinear shunts of the invention are not utilized.

To determine the degree of saturation and thereby the nonlinearity ofthe nonlinear shunts, search coils were wound around the shuntlaminations in the ballast transformer of the illustrated example.During this test, the secondary winding was shorted through a lowimpedance ammeter in order to reduce the llux in the secondary windingto substantially zero. Voltage readings were recorded for various valuesof the primary fluxdensity and the flux density in the shunt wascomputed. Curve 7 C of FIG. 10 represents a plot of primary flux densityin lines of per square inch against shunt flux density in lines of persquare inch. From curve C it will be seen that the metallic shuntcontained a major part of the primary flux until it started to saturateat a shunt density of approximately 90,000 lines per square inch. CurveD,

the dashed lined curve, represents a plot of primary flux density inlines per square inch against leakage reactanee inch. This valuecorresponds with a leakage reactanee of 400 ohms. The leakage reactaneewas approximately 450 ohms at the point in each half cycle when the fluxdensity was at a minimum. Therefore, it will be seen that the leakagereactanee provided by the shunt varies from 450 ohms to 400 ohms duringeach half cycle.

When the nonlinear metallic shunts were removed from the ballasttransformer, the leakage reactanee was found to be substantiallyconstant at approximately 299 ohms. It will be readily apparent to thoseskilled in'the art that the nonlinear shunts provide the ballast circuitdesigner with a versatile tool for improving a ballast apparatus. I

it will be appreciated that although the illustrative em bodiments ofthe invention involve ballast apparatus for starting and operating twoare discharge lamps, it will be apparent that the invention can bereadily embodied in a ballast apparatus for starting and operating asingle lamp. While this invention has been explained by describingparticular embodiments thereof, it will be understood that modificationsmay be madewithout departing from the scope of the invention as definedin the appended claims. I

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An alternating current source, a ballast apparatus for starting andoperating at least one are discharge lamp therefrom comprising amagnetic core having an elongated central winding leg and a yokedefining at least one coil receiving window with said central windingleg, at least one primary winding and at least one lead secondarywinding disposed in an end to end relationship on said central windingleg within said coil receiving windows, a pair of input leads connectedwith the alternating current source, said primary winding beingconnected across said input leads, a first and second output leadconnected in circuit with said secondary winding, a capacitor connectedin circuit with said secondary winding and one of said output leads,said output leads being provided for connection across said lamp, and atleast one nonlinear shunt disposed between said primary winding and saidlead secondary winding and having at least one end spaced from saidmagnetic core.

2' The ballast apparatus of claim 1 wherein said nonlinear shunt iscomprised of at least one relatively thin transverse lamination ofmagnetic material vertically disposed between said primary winding andsaid lead secondary winding. p

3. An alternating current source, a ballast apparatus for starting andoperating at least one are discharge lamp therefrom comprising amagnetic core having a central winding leg and a yoke defining coilreceiving windows with said central winding leg, at least one primarywinding, a pair of input leads connected across the alternating currentsource, said primary winding being connected across said input leads, alag secondary winding mounted on said central winding leg, a linearshunt disposed between said primary winding and said lag secondarywinding, said metallic shunt separating said coil receiving windows intoa first and second seccoil receiving windows, a first and second outputlead connected in circuit with said lead secondary winding, acapaci-tor, said capacitor being in series circuit relationship withsaid lead secondary winding and said first output lead, said first andsecond output leads being provided for connection across said lamp, saidprimary winding being arranged on said central winding leg adjacent tosaid shunt and in an end to end relationship with said lead secondary,a. nonlinear transverse shunt interposed between said primary windingand said lead secondary winding, said nonlinear shunt providing asubstantially variable leakage reactanee and said linear shunt providing'a substantially constant leakage reactanee.

4. The ballast apparatus set forth in claim 3 wherein said nonlinearshunt is comprised of at least one relatively thin lamination ofmagnetic material inserted between said lead secondary winding and saidprimary winding.

5. An alternating current source, a ballast apparatus for starting andoperating a pair of arc discharge lamps therefrom comprising a magneticcore having a central winding leg and a yoke defining coil receivingwindows with said central winding leg, a first primary winding, a secondprimary Winding, a pair of input leads connected across the alternatingcurrent source, said primary windings being connected across said inputleads, a secondary winding mounted on said central winding leg withinsaid coil receiving windows, said first primary winding being disposedin an end to end relationship with one end of said secondary winding andsaid second primary winding being disposed on said central winding legin an end to end relationship at the other end of said secondarywinding, a first output lead connected in circuit with said secondarywinding, a second output lead connected in" circuit with one of saidprimary windings, said output leads being provided for connection acrosssaid lamps, a capacitor connected in series circuit relationship withsaid secondary winding and one of said output leads, said secondarywinding being connected in au-totransformer relationship with one ofsaid primary windings, a first nonlinear shunt interposed between saidfirst primary winding and said secondary winding and having at least oneend spaced from said magnetic core, and a second nonlinear shuntinterposed between said second primary winding and said secondarywinding, both of said nonlinear shunts providing a variable leakagereactance during operation of said transformer. I

6. The ballast apparatus of claim 5 wherein said nonlinear shunt iscomprised of at least one relatively thin lamination of magneticmaterial.

7. An alternating current source, a ballast apparatus for starting andoperating at least one are discharge lamp therefrom comprising amagnetic core having a central winding leg and a yoke defining coilreceiving windows with said central winding leg, a first primarywinding, 9. second primary winding, a pair of input leads connectedacross the alternating current source, said primary windings beingconnected across said input leads, a secondary winding mounted on saidcentral winding leg within said coil receiving windows, a first outputlead connected in circuit with said secondary winding, a second outputlead connected in circuit with one of said primary windings, said outputleads being provided for connection across said lamp, a capacitorconnected in series with said secondary winding and one of said outputleads, saidsecondary winding being connected in autotransformerrelationship with one of said primary windings, said first primarywinding being disposed in an end to end relationship with one end ofsaid secondary 1 winding and said second primary winding being disposedon said central winding leg in an end to end relationship at the otherend of said secondary winding, a nonlinear transverse shunt interposedbetween one of said primary windings and said secondary winding, saidnonlinear shunt providing a variable leakage reactance during operationof said transformer and having at least one end spaced from saidmagnetic core.

8. An alternating current source, a ballast apparatus for starting andoperating a pair of arc discharge lamps therefrom comprising a magneticcore having an elongated central winding leg and a pair of yoke legsdefining coil receiving windows with said central core leg andintegrally formed shunt projections on at least one of said legs, saidshunt projections separating said coil receiving windows into first andsecond sections and having an air gap in the magnetic circuit ,of saidprojections, a lag secondary winding disposed on said central windingleg within said first l 2. section, a primary winding disposed on saidcentral winding leg adjacent to said projections within said secondsection, a pair of input leads connected across the alternating currentsource, said primary winding being connected across said input leads, alead secondary winding disposed on said central winding leg within saidsecond section in an end to end relationship with said primary winding,a pair of output leads for connection across the lamps, a capacitor,said capacitor being connected in series circuit relationship with saidlead secondary winding, said output leads being connected in circuitwith said secondary winding, and a nonlinear shunt interposed betweensaid lead secondary winding and said primary winding and spaced fromsaid central winding leg, said shunt projections and air gap providing asubstantially linear leakage reactance and said nonlinear shuntproviding a variable leakage reactance.

9. The ballast apparatus of claim 8 wherein said nonlinear shunt iscomprised of at least one relatively thin lamination of magneticmaterial.

10. An alternating current source, a ballast apparatus for starting andoperating arc discharge lamps comprising a first primary winding, afirst secondary winding, a first magnetic core, said first primarywinding and said first secondary winding being magnetically coupled onsaid first magnetic core, a first nonlinear shunt interposed betweensaid first primary and secondary windings and spaced from said firstmagnetic core to define an air gap therebetween, a second magnetic core,a second primary and a second secondary winding coupled therewith onsaid second magnetic core, a second nonlinear shunt interposed betweensaid second primary winding and said second secondary winding and spacedfrom said second magnetic core to define an airgap therebetween, a pairof input leads connected across said alternating current source, saidfirst and second primary windings being connected across said inputleads, a first output lead connected in circuit with one end of saidfirst secondary winding, said secondary windings being seriallyconnected, a second output lead connected in circuit with one of saidprimary windings, said output leads being provided for connection acrosssaid lamps, a capacitor connected in series circuit relationship with atleast one of said secondary windings and with one of said output leads,said nonlinear shunts providing a leakage reactance at an early portionof each half cycle of the alternating current supplied at said outputleads, said leakage reactance being decreased during a later portion ofsaid half cycle.

11. An alternating current source, a ballast apparatus for starting andoperating a pair of serially connected arc discharge lamps therefromcomprising a first primary winding and a first secondary winding coupledtherewith, a first magnetic core, said first primary winding and firstsecondary winding being mounted on said first magnetic core, a firstnonlinear shunt interposed between said first primary winding and saidfirst secondary winding and spaced from said first magnetic core todefine an air gap therebetween, a second magnetic core, a second primarywinding and a second secondary winding-coupled therewith on said secondmagnetic core, a second nonlinear shunt interposed between said secondprimary winding and said second secondary winding and spaced from saidsecond magnetic core to define an air gap therebetween, a pair of inputleads connected with the source of alternating current, said first andsecond primary windings being connected across said input leads, a firstoutput lead being connected in circuit with an end of one of saidsecondary windings, said secondary windings being serially connected, asecond output lead being connected in circuit with one of said primarywindings, said output leads being provided for connection across saidserially connected lamps, a capacitive reactance connected in serieswith one of said secondary windings to provide a net capacitivereactance in the secondary circuit, a starting capacitor connected incircuit with one of said output leads for connection across one of saidlamps, said nonlinear shunts providing a leakage reactance at an earlyportion of each half cycle of the alternating current supplied at saidoutput leads, said leakage reactance being decreased during a laterportion of said half cycle.

12. The ballast apparatus set forth in claim 11 in which said firstsecondary winding is connected in auto-transformer relationshipwith saidfirst primary winding and in series circuit relationship with saidsecond secondary winding.

13. An alternating currentsource, a ballast apparatus for starting andoperating a pair of arc discharge lamps therefrom comprising a pair ofserially connected are discharge lamps, a magnetic core, a primarywinding, a first secondary winding, a second secondary Winding,-saidfirst and second secondary windings being magnetically coupled with saidprimary winding on said magnetic core, a pair of input leads connectedwith an alternating current supply, said primary winding being connectedacross said input leads, a starting circuit including one of said lampsand at least said first secondary winding for applying igniting voltageto said lamp, and an operating circuit ineluding said serially connectedlamps, said primary winding and said second secondary winding forapplying an operating voltage to both of said lamps, a linear shunthaving an air gap and interposed between one end of said primary Windingand said first secondary winding, a nonlinear shunt interposed betweenthe other end of said primary winding and said second secondary windingand spaced from said magnetic core to define an air gap therebetween, acapacitor connected in both said starting cir-r cuit and said operatingcircuit, said first secondary winding having suflicient inductivereactance to prevent substantial current flow through the winding duringthe normal operation of said lamps, said lamps being operated by thevoltage across the primary and said second secondary winding and saidnonlinear shunt providing a leakage reactance at an early portion ofeach half cycle of the alternating current supplied to said lamps, saidleakage reactance being decreased during the later portion of said halfcycle. 1

14. The ballast apparatus as set forth in claim 13 in which said firstsecondary winding is in autotransformer relationship with said primarywinding and said second secondary winding connected in series circuitrelationship with said first secondary.

15. An alternating current source, a ballast apparatus :for operatingand starting arc discharge lamps therefrom comprising at least onemagnetic core, at least one primary winding and at least one leadsecondary windingmagnetically coupled on said magneticcore, a transversenonlinear shunt interposed between said primary and said secondarywindings and spaced firom said magnetic core to define an air gaptherebetween, a pair of input leads connected with the alternatingcurrent source, said primary winding being connected across said inputleads, a pair of output leads for applying the voltage of at least oneof said lead secondaries across said lamps, one of said output leadsbeingconnected in circuit with one of said input leads and the other ofsaid output leads being connected in circuit with one end of saidsecondary winding, a capacitive reactor connected in series circuitrelationship with said secondary winding and one of said output leads,said nonlinear shunt providing a leakage reactance at an early portionof each half cycle of the alternating current at said output leads, saidleakage reactance being decreased during a later portion of said halfcycle. r

16. The ballast apparatus as set forth in claim 15 in which saidsecondary winding is connected in autotransformer relationship with saidprimary winding.

17. An alternating current source, a ballast apparatus :for operating atleast a pair of serially connected are discharge lamps therefromcomprising at least a pair of serially connected arc discharge lamps, amagnetic core, a first primary winding, a second primary winding and alead secondary winding, said I windings being magnetically coupled onsaid magnetic core, said first primary winding being disposed at one endof said lead secondary winding and said second primary winding beingdisposed on the other end of said secondary winding on said magneticcore, a first nonlinear shunt being interposed between said leadsecondary winding and said first primary winding and spaced from saidmagnetic core to define an air gap therebetween, a second nonlinearshunt being interposed between said lead secondary winding and saidsecond primary winding and spaced from said magnetic core to define anair gap therebetween, a

pair of input leads connected with the alternating current source, saidfirst and second primary windings being connected across said inputleads, a pair of output leads for applying the voltage of at least saidlead secondary winding across said serially connected lamps, at leastone starting capacitor connected in circuit with one of said outputleads for starting at least one of said lamps, a capacitive reactorconnected in series circuit relationship with said secondary winding andone of said output leads, said reactance being sufiicient to render thenet reactance of said secondary circuit capacitive and said nonlinearshunts providing a leakage reactance at an early portion of each halfcycle of the alternating current supplied at said output leads, saidleakage reactance being decreased during a later portion of said halfcycle.

'18. The ballast apparatus as set forth in claim 17 wherein said leadsecondary winding is connected in autotransformer relationship with saidfirst primary winding across said output leads.

References Cited in the file of this patent UNITED STATES PATENTS

